Active pullup circuitry for open-drain signals

Abstract

Circuitry and methods are provided for reducing rise time associated with signals on an open-drain or open-collector signal line. Signal line voltage is monitored to determine if the signal line is being pulled LOW. If the signal line is not being pulled LOW, as indicated by signal line voltage exceeding a threshold level, additional pullup current is provided. The additional current may be provided gradually in relation to the signal line voltage, or may be provided in full whenever voltage exceeds the threshold. Circuitry may also be provided to monitor voltage slew rate on the signal line, and to enable the additional pullup current only when the slew rate exceeds a positive threshold level.

What is claimed is:  
     
       1. Pullup circuitry for an open-drain signal line, the pullup circuitry comprising: 
       circuitry for monitoring a slew rate of a signal on the open-drain signal line;  
       circuitry for providing a pullup current to the open-drain signal line responsive to the monitored slew rate exceeding a slew rate threshold; and  
       circuitry for increasing the pullup current responsive to a voltage level of the signal.  
     
     
       2. Pullup circuitry for an open-drain signal line, the pullup circuitry comprising: 
       circuitry for monitoring a slew rate of a signal on the open-drain signal line;  
       circuitry for providing a pullup current to the open-drain signal line responsive to the monitored slew rate exceeding a slew rate threshold, wherein the circuitry for providing the pullup current comprises first circuitry for providing a first current independent of the monitored slew rate, and second circuitry for providing a second current responsive to the monitored slew rate; and  
       circuitry for increasing the pullup current responsive to a voltage level of the signal.  
     
     
       3. The pullup circuitry of  claim 2  wherein the pullup current is increased responsive to the voltage level exceeding a voltage threshold, and wherein the increase in pullup current is a function of the voltage level relative to the voltage threshold. 
     
     
       4. Pullup circuitry for an open-drain signal line, the pullup circuitry comprising: 
       circuitry including an operational amplifier for monitoring a slew rate of a signal on the open-drain signal line;  
       circuitry for providing a pullup current to the open-drain signal line responsive to the monitored slew rate; and  
       circuitry for increasing the pullup current responsive to a voltage level of the signal.  
     
     
       5. Pullup circuitry for an open-drain signal line, the pullup circuitry comprising: 
       circuitry including an operational amplifier for monitoring a slew rate of a signal on the open-drain signal line;  
       circuitry for providing a pullup current to the open drain signal line responsive to the monitored slew rate, wherein the circuitry for providing the pullup current comprises first circuitry for providing a first current independent of the monitored slew rate, and second circuitry for providing a second current responsive to the monitored slew rate; and  
       circuitry for increasing the pullup current responsive to a voltage level of the signal.  
     
     
       6. The pullup circuitry of  claim 5  wherein the pullup current is increased responsive to the voltage level exceeding a voltage threshold, and wherein the increase in pullup current is a function of the voltage level relative to the voltage threshold. 
     
     
       7. The pullup circuitry of  claim 6  wherein the pullup current is increased responsive to the monitored slew rate exceeding a slew rate threshold. 
     
     
       8. The pullup circuitry of  claim 4  wherein the circuitry for monitoring the slew rate is a differentiator. 
     
     
       9. A method for biasing an open-drain signal line, the method comprising: 
       monitoring a slew rate of a signal on the signal line;  
       providing a pullup current to the signal line responsive to the monitored slew rate exceeding a slew rate threshold; and  
       increasing the pullup current responsive to a voltage level of the signal.  
     
     
       10. A method for biasing an open-drain signal line, the method comprising: 
       monitoring a slew rate of a signal on the signal line;  
       providing a pullup current to the signal line responsive to the monitored slew rate exceeding a slew rate threshold, wherein providing the pullup current comprises providing a first current independent of the monitored slew rate, and providing a second current responsive to the monitored slew rate; and  
       increasing the pullup current responsive to a voltage level of the signal.  
     
     
       11. The method of  claim 10  wherein the second current is provided responsive to the monitored slew rate exceeding a slew rate threshold.